1. Field of the Invention
The present invention relates to an image forming method utilizing electrophotography and electrostatic recording, to a process cartridge and to an image forming apparatus. In particular, the present invention relates to an image forming method, a process cartridge and an image forming apparatus using a compound having an acid-adsorbing ability.
2. Description of the Related Art
The Karlson method has been generally used when an image is formed in copier or a laser beam printer. In conventional image forming methods, an image is formed by developing an electrostatic latent image formed on a photoreceptor by optical means, transferring the electrostatic latent image to an image receiving member such as recording paper, and next fixing to the image receiving member using heat and pressure. Because the photoreceptor is used repeatedly, a cleaning device is disposed to remove residual toner left on the photoreceptor after the transfer.
A structure referred to as a function-separating type, in which a charge generation layer is isolated from a charge transfer layer, has been devised and put to practical use in recent years as an electrophotographic photoreceptor in view of sensitivity and stability. Electrophotographic photoreceptors having such a structure comprise two layers consisting of a charge generation layer, which is produced by binding a charge generation material using a suitable resin as a binding material (binder resin), and a charge transfer layer, which is produced by dispersing or dissolving a charge transfer material in a binder resin. The layer containing a charge transfer material contains a positive hole transfer material in many cases. As the binder resin, thermoplastic resins such as polycarbonate resins, polyester resins, acryl resins and polystyrene resins, and heat-curable resins such as polyurethane resins and epoxy resins are under study.
In this case, the surface of the charge transfer layer must be negatively charged by corona charging or roller charging. This gives rise to problems in that the characteristics of the photoreceptor are adversely affected due to various causes, such as resin deterioration caused by ozone generated when the charge surface layer is negatively charged, wear, reduced sensitivity and reduced charging ability caused by the electrical impact of discharging at the photoreceptor surface, and mechanical breakdown resulting from friction during subsequent toner development, transfer to paper, and cleaning.
Various studies such as those listed below have been made in relation to the foregoing problems. Attempts have made to blend a polysiloxane resin with a copolymer component or other resins, and studies have been made with respect to improve the quality, life and cleaning characteristics of photoreceptors using the characteristics of polysiloxane, as can be seen in Japanese Patent Application Laid-open (JP-A) No. 61-238062, which discloses a photoreceptor that uses a heat-curable resin containing a polysiloxane resin for a charge transfer layer; in JP-A No. 62-108260, which discloses a photoreceptor including a protective layer containing a polysiloxane resin; in JP-A No. 4-346356, which discloses a photoreceptor disposed with a protective layer formed by dispersing silica gel, a urethane resin or a fluororesin in a heat-curable polysiloxane resin; and in JP-A No. 4-273252, which discloses a photoreceptor in which a resin obtained by dispersing a heat-curable polysiloxane resin in a thermoplastic resin is used for a protective layer or as a charge transfer binder resin.
Although polysiloxane has excellent thermal and mechanical strength, it is quite incompatible with organic compounds that function as electronic devices. For this reason, studies have been with respect to photoreceptors in which a charge transfer material having an unsaturated bond is bound directly with polysiloxane such as poly(hydrogen methylsiloxane) to make a resin, which is used as a binder resin for a protective layer or charge transfer material (JP-A No. 8-319353); photoreceptors in which a thin film produced using a sol gel method is used as a protective layer (Proceedings of IS and T""s Eleventh International Congress on Advances in Non-Impact Printing Technologies, pp. 57-59); and photoreceptors in which an organic silicon modified positive positive hole transfer compound obtained by directly introducing silicon having a hydrolyzable group into a charge transfer material is used for an electrophotographic photoreceptor (JP-A No. 9-190004). In the photoreceptors described in Proceedings of IS and T""s Eleventh International Congress on Advances in Non-Impact Printing Technologies, pp. 57-59, and in JP Nos. 2575536 and 9-190004, a firm film is formed because siloxane forms a three-dimensional network. As a result, these photoreceptors have attracted considerable attention because mechanical strength is largely improved.
As disclosed in JP-A Nos. 11-38656, 11-184106 and 11-316468, we developed novel materials previously and demonstrated that these materials have superior characteristics. We found that when a series of these materials is used as the surface layer of an electrophotographic photoreceptor, the surface layer had overwhelmingly superior thermal and mechanical strength with respect to conventional surface layers, whereby deterioration of the surface layer caused by wear can be significantly reduced and longevity can be improved.
However, it was found that when the surface layer is used for a long period of time, especially in a humid environment, image defects including image flow are caused.
As a result of investigating the cause of this problem, the following is surmised. It is known that, when a photoreceptor is charged by charging means such as corona charging or a conductive roller, discharge products (active products) such as ozone and NOx are produced in the process. Ozone and NOx produced in the above step not only pose a problem in terms of environmental sanitation, but they also act on the surface of the photoreceptor to increase potential fluctuation and residual potential, and impact photographic characteristics and images (e.g., image flow), thus reducing the durability of the photoreceptor. Therefore, the surface of the photoreceptor is occasionally denatured by the action of the ozone and NOx. Moreover, when the surface of the photoreceptor is hydrophilic, ozone and NOx adhere to the surface, whereby moisture in the atmosphere also tends to adhere to the surface, with the result being that electrical resistance of the surface is microscopically reduced and it is difficult to maintain the charge generated by the charging.
The surface of the photoreceptor comprising the aforementioned series of materials has overwhelmingly superior mechanical strength and significantly small abrasion loss. On the other hand, a conventional surface layer is abraded to some extent. Taking this phenomenon into account, it is surmised that a certain degree of abrasion of the surface layer can suppress the renewal of a deteriorated surface and the progress of the adhesion of products created by discharge. Accordingly, it is believed that it is difficult for the aforementioned phenomenon (suppression the adhesion of products created by discharging) to occur and easy for image defects such as image flow to be generated on a surface layer that has superior mechanical strength and small abrasion loss.
Various studies have been made to suppress these image defects. For instance, a method in which fine particles (abrasives) having an abrasive function are incorporated into a developing agent for the purpose of properly abrading the surface of a photoreceptor (JP-A No. 5-188630) and a method in which a thin film of a fatty acid metal salt is formed on the surface of a photoreceptor to protect the surface layer from adverse effects of discharge products (JP-A No. 2001-5207) have been proposed. Also, for example, a method in which a hydrotalcite compound that adsorbs anions is incorporated into a developing agent to remove discharge products (JP-A No. 2-166461) has been proposed.
However, if the particle diameter of an abrasive is small in the method in which the abrasive is used, abrasive loss is reduced because of small abrasive effects and image defects cannot be sufficiently suppressed. When the particle diameter is large, scratches are caused on the surface of the photoreceptor in the direction of rotation and lines resulting from these scratches appear on the image. Moreover, adhesion (contamination) of toner components resulting from these scratches progresses, and black points, white points and black lines resulting from the adhesion appear on the image.
In the method in which a thin film of a fatty acid metal salt is formed on the surface of a photoreceptor, the coefficient of friction decreases and cleanability is improved when the surface of the photoreceptor is cleaned in a cleaning step with a rubber blade such as a urethane blade. However, because the coefficient of friction with the photoreceptor having the surface layer resistant to abrasion rises, leading to a rise in the rotational torque of the photoreceptor, the blade end pressed to the photoreceptor is abraded or chipped, with the result being that black lines caused by cleaning inferiors appear on an image.
Moreover, in the method in which a hydrotalcite compound is incorporated into a developing agent to remove discharge products, adhesion (contamination) of the hydrotalcite compound resulting from irregularities and scratches caused by partial wear on the surface of the photoreceptor is easily caused, even though this method has initial effects. Therefore, black points, white points and black lines resulting from the adhesion appear on the image in the case of a conventional photoreceptor.
Methods of developing this electrostatic latent image include a one-component developing method, which uses only a toner, and a two-component developing method, which uses a toner and a carrier. In the case of a two-component developing agent in the two-component developing method, the toner and the carrier are stirred to frictionally charge the toner. Therefore, the amount of frictional charge of the toner can be controlled to a considerable extent by selecting carrier characteristics and stirring conditions. Thus, image quality is highly reliable and excellent.
The toner used in the electrophotographic process is usually produced by adding various resins (e.g., polyester resin, styrene-acryl resin, and epoxy resin), colorants, charge control agents, releasing agents and the like, and then melting, kneading, and uniformly dispersing the same, following by crushed the mixture into a predetermined grain size and removing excessively coarse powders and micropowders using a classifier. However, it has become necessary to further reduce toner grain size along with the demand for higher image quality in recent years. It has also, in view of the demand to reduce energy, become necessary to lower the transition temperature and softening point of resins in order to achieve fusing at lower temperatures.
With respect to color toners used in full-color copiers and printers, different color toners must be mixed sufficiently in a fusing step, and color reproducibility and the transparency of overhead projector (OHP) images are essential. Generally, these color toners are preferably formed using a sharp-melt low molecular resin in order to raise color-miscibility in comparison with black toner.
Conventionally, waxes such as polyethylene and polypropylene, which have high crystallinity and a relatively high melting point, are used,in black toner to obtain offset resistance for fusing. However, these waxes compromise the transparency of overhead projector images in full-color toner. For this reasons ordinary full-color toner contains no wax, and a method has been adopted in which silicon rubber or a fluororesin, which is highly releasable with respect to toner, is used to form,the surface of a heat-fusing roller, and a releasable liquid such as silicon oil is supplied to the surface to prevent offset. This method is very effective in terms of preventing the offset phenomenon of toner, but there is a problem in that it requires a device for supplying the offset-preventing liquid. This runs counter to the need to reduce the size and weight of copiers and printers. Moreover, the offset-preventing liquid exudes an unpleasant odor due to being vaporized by heat, and can sometimes contaminate the machine.
Therefore, studies are being made as to toners that are produced by a kneading and crushing method, comprise a sharp-melt resin, a colorant and a low-melting point wax, and have a small grain diameter. In this kneading and crushing method, a thermoplastic resin and the like are melted and kneaded together with a pigment, a charge control agent, a releasing agent such as wax; and then the melted and kneaded mixture is micronized and classified after being cooled to produce a desired toner.
However, in the case of a toner produced by the kneading and crushing method, generally its shape is undefined and its surface composition is not uniform. Although, in this method, the shape and surface composition of the toner are changed subtly corresponding to the crushing characteristics of materials to be used and conditions in a crushing step, it is difficult to control these characteristics in desired ranges intentionally. When the shape of the toner particles is undefined, only insufficient fluidity is obtained even if a fluidity adjuvant is added and fine particles of the fluidity adjuvant are moved to recesses in the toner particles and embedded in the recesses by mechanical force such as shearing force, giving rise to the problem that fluidity is lowered with time and developability, transferability and cleaning ability are impaired.
In light of this, studies being are made with respect to a suspension polymerization method and an emulsion polymerization coagulation method as methods for producing spherical toners that cannot be easily obtained by the above kneading and crushing method.
In the suspension polymerization method, a polymerizable monomer is dispersed in an aqueous medium together with a colorant and a releasing agent, and then the polymerizable monomer is polymerized to obtain a toner.
In the emulsion polymerization coagulation method, a resin dispersion is prepared by emulsion polymerization, and a colorant dispersion in which a colorant is dispersed in a solvent, and a dispersion in which a releasing agent is dispersed, are separately prepared. These dispersions are mixed to form coagulated particles having a particle diameter corresponding to that of a toner, and then fused by being heated to thereby obtain a toner. According to this emulsion polymerization coagulation method, the shape of toner particles can be arbitrarily controlled, from an undefined shape to a spherical shape, by selecting heating temperature conditions
Studies are also being made with respect to a carrier having a small particle diameter in order to stably charge toner particles having a small particle diameter. These proceed from the fact that the surface area of the carrier must be increased, because the surface area of the toner particles increases when the toner particles have a small particle diameter. Additionally, a ferrite core having a smaller specific gravity than iron powder, a magnet-dispersion carrier containing a resin as a constitutional component, and a polymerized carrier are being studied. This is because the running torque of a developing machine can be made small by decreasing the mass of a developing agent.
It is an object of the present invention to provide an image forming method, a process cartridge and an image forming apparatus with which an electrophotographic image having superior image quality and fixing ability over a long period of time is obtainable.
It is also an object of the invention to provide an image forming method, a process cartridge and an image forming apparatus with which good cleaning characteristics are secured and an electrophotographic image that remains good even in a hot and humid environment is obtainable.
The above objects of the invention are attained by the invention shown below.
According to a first aspect of the invention, there is provided an image forming method comprising:
developing, with a developing agent, an electrostatic latent image formed on a surface of a photoreceptor to form a toner image;
transferring the toner image onto an image receiving member to form a transferred image; and
fixing the transferred image onto the image receiving member to form an image,
wherein the photoreceptor includes a layer that contains a siloxane compound having charge-transferability and a crosslinking structure, with a compound having acid-adsorbing ability being supplied to the surface of the photoreceptor.
According to a second aspect of the invention, there is provided an image forming method, wherein shape factors SF-1 and SF-2 of the toner respectively satisfy expressions (1) and (2), and the average particle diameter of the toner is 3 xcexcm or more and 11 xcexcm or less:
100xe2x89xa6SF-1xe2x89xa6140xe2x80x83xe2x80x83(1)
100xe2x89xa6SF-2xe2x89xa6120xe2x80x83xe2x80x83(2)
provided that SP-1=(maximum length of diameter)2xc3x97100xcfx80/4 and SF-2=(peripheral length of projected image)2xc3x97100/4).
According to a third aspect of the invention, there is provided a process cartridge used in the image forming method, the process cartridge comprising at least:
a photoreceptor including a layer that contains a siloxane compound having charge-transferability and a crosslinking structure; and
supply means for supplying a compound having acid-adsorbing ability to a surface of the photoreceptor.
According to a fourth aspect of the invention, there is provided an image forming apparatus comprising a photoreceptor, latent image forming apparatus for forming an electrostatic latent image formed on a surface of the photoreceptor, a developing device for developing the latent image using a toner, and a transfer device for transferring the toner image to an image receiving member, wherein the photoreceptor includes at least
a layer that contains a siloxane compound having charge-transferability and a crosslinking structure, and
supply means for supplying a compound having acid-adsorbing ability to the surface of the photoreceptor.
According to a fifth aspect of the invention, there is provided an image forming apparatus, wherein shape factors SF-1 and SF-2 of the toner respectively satisfy expressions (1) and (2), and the average particle diameter of the toner is 3 xcexcm or more and 11 xcexcm or less:
100xe2x89xa6SF-1xe2x89xa6140xe2x80x83xe2x80x83(1)
xe2x80x83100xe2x89xa6SF-2xe2x89xa6120xe2x80x83xe2x80x83(2)
provided that SF-1=(maximum length of diameter)2xc3x97100xcfx80/4 and SF-2=(peripheral length of projected image)2xc3x97100/4).